Automatic dialing systems, which have found increasing commercial application in areas such as operator direct sales and the delivery of prerecorded messages, often include detection means with which to monitor the progress of the dialed call. Such detection means must be capable of distinguishing between the call progress tones emitted by a "busy" signal, a dial tone and a ringing telephone and the signals generated by vocal input. Although the range of frequencies for the call progress tones is generally between 350-620 Hz, monitoring the signals is additionally complicated by the fact that human vocal signals can fall within the same frequency range, though seldom would remain at the same frequency throughout a series of successively emitted signals. The various call progress tones have different periods, for example, a "busy" signal tone endures for a brief period followed by a period during which there is, ideally, no detectable signal, whereas a "ring" signal is emitted for a period of longer duration than that of the busy signal and is followed by a longer period during which there is no detectable signal. Call progress circuitry must therefore be equipped to detect, characterize and respond to the variously emitted tones. Call progress detection circuitry must also be readily incorporated into the operating environment, requiring it to be compact. In addition, although there are currently standardized identifiable tones ranges, the established call progress tone frequencies may be varied in the future to accommodate increasingly sophisticated equipment and services. Call progress detection circuitry should also, ideally, be adaptable to new frequency ranges.
In the past, signal filtering systems have been provided to automatic dialing equipment for the monitoring of specific frequencies of incoming call progress signals. A simple detection circuit of the prior art includes a bandpass filter centered on the call progress frequency range of 350-620 Hz. Such a wide range filter will, however, falsely detect voice signals falling within the relevant frequency range. Yet another, more elaborate, call progress detection system includes a series of narrow bandpass filters at each of the relevant frequencies of the known call progress tones, 350, 440 480 and 620 Hz Although the latter filtering system minimizes the possibility of misinterpreting a voice signal as a call progress tone, it is limited in its ability to both receive and correctly identify signals outside of its narrow filter ranges.
All of the prior art circuitry detects call progress signals by the presence of energy in its pass band. Distinguishing between false triggering, such as in the instance of voice signals in the bandpass range, is not possible in such an "either/or" (i.e., energy in the pass band or not) system.
It is therefore an objective of the present invention to provide an improved call progress detection circuit for incorporation with automatic telephone dialing equipment.
It is another objective of the present invention to provide a call progress detection circuit which is capable of detecting and identifying signals in a broad frequency range.
It is yet another objective of the present invention to detect the presence of call progress tones to the exclusion of voice signals in the same frequency range, using more than just the energy in a pass band to detect same.
It is still another objective of the present invention to provide apparatus for distinguishing between call progress tones and human voice signals input to the apparatus.
Yet another objective of the present invention is to provide apparatus for incorporation into an automatic telephone dialing system which is compact.
Still another objective of the present invention is to provide call progress detection apparatus on an integrated circuit chip.
It is a further objective of the invention to provide call progress detection circuitry which can not only detect but also specifically identify the input call progress tone and which is adaptable to newly-established tone frequency ranges.